You understand the Japanese text correctly, and you're right in that there is certain ambiguity (like, does someone who has been smoking once a month for the past 10 months really count?). But the sentence works as it is. (If I were in charge, I might slightly change the wording, though...)
The key observation here is that this health check question is meant for stratify the questionees according to the risks and help to interpret the results: the true question is "Are you [your health] under the influence of smoking?". As such, they exclude those who are barely (sort of) smoking, an arbitrary and rough criteria of which is less than 100 cigarettes AND less than 6 months. So if I started smoking 2.5 months ago and this is the 95th cigarettes I smoke today, I should probably think twice before I check "No". Same goes for the population who smoke longer than 6 months but the consumption is (from a commonsense view) very low. You think twice, check the choice which you feel more suitable, and if you have a chance, tell your situation to the doctor directly.
Another point is that smoking, when asking about your habits, implies smoking regularly.
This sets certain lower bounds implied to the number of cigarettes consumed when one think of themselves as "smoking for more then 6 months".
Additionally, I feel that there are much less percentage of smokers who smokes only occasionally than social drinkers are in drinkers (the choices for your drinking habits usually has 機会飲酒, roughly social drinking). Therefore the number of edge-cases here is, if I may guess, epidemiologically small.
I think it being written in another language than your native one brought you unnecessary suspicion. You can be confident in your skill: the question indeed leaves some space for interpretation, which does not spoil the value of the question practically.